This invention relates generally to methods and apparatus for reducing electromagnetic interference (`EMI`) emissions from PC board traces in a digital system, and more particularly for reducing EMI emissions from clock and other synchronous signal traces for a computer or other digitally-clocked system.
Electromagnetic interference is electromagnetic energy emitted from electronic devices which, either directly or indirectly, contributes to a degradation in performance of an electronic receiver or other electronic system. EMI radiation from poorly shielded electronic devices, for example, degrade radio and television signals resulting in audible or visible static at receivers picking up such signals. Governments typically regulate EMI emissions to enhance public use of the radio wave spectrum and other electromagnetic wave spectra. In the United States, for example, the F.C.C. requires testing of devices and rates the devices by class according to their emissions. The United States F.C.C. Agency rates EMI emissions over a 120 kilohertz ("kHz") bandwidth. The 120 kHz bandwidth corresponds to the typical bandwidth of a conventional communication receiver, such as an FM receiver. Reduced EMI emissions within such bandwidth reduce the interference output, otherwise perceived by a listener or viewer as, for example, static or white noise.
Typical precautions taken by electronic manufacturers are to provide shielding of electronic devices to minimize EMI emissions. Computer manufacturers, for example, typically use shielded cables and shielded housings to minimize EMI emissions. This invention is directed toward a digital method and apparatus for reducing detectable EMI emissions within a critical bandwidth (e.g., 120 kHz).